1. Field of the Invention
This invention concerns a method of elimination of the phenomenon of pumping of a motor vehicle with automatic transmission, as well as a motor vehicle employing that method.
2. Discussion of the Background
The pumping phenomenon is common in the field of motor vehicles with automatic transmission. It involves an instability of the automatic transmission, which in some circumstances changes constantly from one ratio to the higher ratio and vice versa.
This phenomenon generally occurs when a vehicle equipped with an automatic transmission is subjected to a certain level of resistant forces caused, in practice, by a sloping road, a presence of very strong wind, a sizable load of the vehicle, or even when the vehicle is traveling at a high altitude, that is, under relatively weak atmospheric pressures.
A theoretical solution to this problem consists in preventing any shift to the higher gear ratio if the acceleration of the vehicle on that higher ratio would be negative, and if the vehicle is subjected to strong resistant forces.
A partial solution to the pumping problem was introduced, in practice, by the method of adaptive scheduling of shifting down, described in patent U.S. Pat. No. 5,241,476 of the Chrysler Corporation. According to that document, the method consists in determining whether the vehicle could maintain its speed on the higher ratio and, if not, preventing the shift from the higher ratio. That method resorts to the calculation of acceleration of the vehicle on the higher ratio, but without precisely knowing the weight of the vehicle. The result of the determination is therefore erroneous. Furthermore, according to that document, only cases of pumping between third and fourth gear are eliminated, while the other cases of pumping can occur.
In addition, this method consists in locking the ratio engaged if the calculations lead to an acceleration that would be negative on the higher ratio. That criterion leads to untimely lockings on the ratio engaged, once the acceleration value calculated is slightly negative. Likewise, if acceleration ranges around zero, it would be randomly switched from a nonlocking condition to a locking condition, and so on.